1. Field of the Invention
The present invention generally relates to the field of wireless communication and more particularly, the present invention relates to the methods for preventing HFN de-synchronization due to the interaction between RESET procedure and re-establishment procedure in the RLC layer which handles the ciphering functionality, of a wireless communication system like 3GPP UMTS standards.
2. Description of the Related Art
Universal Mobile Telecommunications System (UMTS) is a third generation (3G) mobile communication system developed in Europe. The UMTS employs Wideband Code-Division Multiple Access (WCDMA) as a radio access technique. WCDMA is a Direct Sequence Code Division Multiple Access (DS-CDMA) technique of a Frequency Division Duplex (FDD) scheme, which transmits data using a bandwidth of 5 MHz in a frequency band of 2 GHz.
Generally, it has been defined that the ciphering and deciphering of data in the UMTS system are performed by a Radio Link Control (RLC) layer. Therefore, in the UMTS system, when a problem related to ciphering and/or deciphering of data occurs upon transmission/reception of data, the interaction of a RESET procedure and the re-establishment procedure can cause Hyper Frame Number (HFN) de-synchronization between sender and receiver of RLC entities. Therefore, with such a service environment, wireless communication systems currently require a method for preventing HFN de-synchronization in the RLC layer.
FIG. 1 illustrates a UMTS radio interface protocol architecture layer proposed for a 3G wireless network and is reproduced from the 3GPP TS 25.301.
Referring to FIG. 1, Layer-1 (L1) or the physical layer of the UMTS radio interface is responsible for coding and modulation of data transmitted over the air. Layer-2 (L2) or the data link layer is subdivided into a RLC sublayer and a Media Access Control (MAC) sublayer. The service provided by layer-2 is referred to as the radio bearer. The control plane radio bearers, which are provided by RLC to Radio Resource Control (RRC), are denoted as signaling radio bearers. The separation in MAC and RLC sublayers is motivated by the need to support a wide range of upper layer services, and also the requirement to provide high efficiency and low latency data services over a wide performance range, i.e. from 1.2 Kbps to greater than 2 Mbps. Other motivators are the need for supporting high Quality of Service (QoS) delivery of circuit and packet data services, such as limitations on acceptable delays and/or data Bit Error Rate (BER), and the growing demand for advanced multimedia services. Each multimedia service has different QoS requirements. The data link layer also includes C-plane signaling and U-plane information for separating the information from control signals.
The RLC layer receives data packets from the higher layers, such as Internet Protocol (IP), through Service Access Point (SAP), and delivers RLC frames to the MAC sublayer. The RLC layer provides three types of SAPs, one for each RLC operation mode, i.e., unacknowledged mode (UM), acknowledged mode (AM), and transparent mode (TM). The RLC frames are queued into logical channels. At the MAC sublayer, the RLC frames are multiplexed onto transport channels. The transport channels are the interface of the physical layer to the data link layer. In fact, data link layer functions are divided in two parts, Physical Layer Independent Convergence Function (PLICF) handled in the RLC, and Physical Layer Dependent Convergence Function (PLDCF) included in the MAC. It is assumed that there is one instance of RLC for each data application/session.
More precisely, a RLC entity is included in the UE and a corresponding RLC entity is included at the UTRAN (network). The operation of the RLC entity at the UE and the network is identical.
RLC layer receives a unit of data often called payload, from the upper layers. This unit of data, or payload, is called a Service Data Unit (SDU). RLC adds control information, called header, for the peer layer to process it. The header combined with the payload is called a Packet Data Unit (PDU).
An AM RLC entity consists of a transmitting side and a receiving side, where the transmitting side of the AM RLC entity transmits RLC PDUs and the receiving side of the AM RLC entity receives RLC PDUs. Whereas a UM or TM RLC entity is either transmitting or receiving type.
The RLC layer is responsible for reliable transmission of the SDUs received from the upper layer. To achieve reliability, a feedback mechanism is used for notifying the transmitter about the reception of the PDUs. It is possible that an RLC PDU may not be received correctly by the peer receiving RLC entity. In such cases, the PDU needs to be retransmitted. The numbers of retransmissions are limited to a preconfigured value MAXDAT. Apart from the feedback on the reverse link, some additional feedback mechanism may be required to report an error detection and if required to do a RESET of the protocol parameters. This feedback may be required on the forward link as well as on the reverse link. All the feedback mechanisms together constitute the control procedures of the RLC. The control procedures are carried out by exchanging control PDUs. An RLC RESET procedure is performed by exchanging RESET and RESET ACK control PDUs between the peer RLC entities.
FIG. 2 illustrates an elementary RLC RESET procedure as defined by the 3GPP RLC protocol specification. Basically, a sender sends a RESET to a receiver. The receiver should respond with a RESET acknowledgement to the sender. The RLC RESET procedure is used to RESET two RLC peer entities, which are operating in an AM. During the RESET procedure, the HFN in UTRAN and UE are synchronized. Two HFNs used for ciphering need to be synchronized, i.e., a DownLink (DL) HFN in the downlink and an UpLink (UL) HFN in the uplink. The RESET procedure can be initiated by either sender or receiver when one of the RESET conditions has occurred, i.e., if “no discard after MAXDAT number of transmissions” is configured and the number of retransmission attempts equals the value MAXDAT, or a status PDU with erroneous sequence number is received.
The other procedure used for initialization of peer RLC entities is RLC re-establishment, which is performed upon request by upper layers. The RLC re-establishment is applicable for AM or UM RLC entity. For UM, the whole RLC entity is re-established. For AM, upper layers may request re-establishment of the whole RLC entity or only the transmitting or receiving side of the RLC entity.
FIG. 3 illustrates the data structure of a regular AM RESET PDU and a regular RESET ACK PDU. A typical PDU includes a number of bytes (octets), where various bit-size fields are defined. For example, as illustrated in FIG. 3, the one-bit D/C field indicates whether the type of an AM PDU is a data or a control PDU. The 3-bits PDU type field indicates what kind of control type the PDU is, i.e., status, RESET, or RESET ACK PDU. The 1-bit Reset Sequence Number (RSN) is used to indicate the sequence of the transmitted RESET PDU. If this RESET PDU is a retransmission of an original RESET PDU, the RSN value is same as the original RESET PDU. Otherwise, the RSN value is changed to the next RSN value. The initial value of this field is zero. The value will be reinitialized every time the RLC is re-established, but will not be reinitialized when the RLC is RESET. RSN field in the RESET ACK PDU is filled with the same value as in RSN field of the received RESET PDU. The 3-bits R1 field is reserved for future functions. The 20-bits HFNI field is used to indicate the HFN, which helps to track the synchronization between a sender and a receiver.
A sender (or receiver) can be a UE or a UTRAN.
The PAD field is used to make ensure a minimum length of the PDU.
In general, a transmission from the UE to the UTRAN is referred to as an UL and the transmission from the UTRAN to the UE is referred to as a DL. Under certain conditions in an AM, either a sender or a receiver will initiate a RESET procedure if one sends too many retries, the number of retries has exceeded the maximum number of retransmission, or one receives a PDU with erroneous sequence number.
As per current 3GPP UMTS standards, when a RESET procedure has been initiated, it can only be ended upon reception of a RESET ACK PDU with the same RSN value as in the corresponding RESET PDU, upon request of re-establishment due to request of re-establishment (for the whole RLC entity or for only the transmitting or receiving side of the RLC entity), or release from upper layer.
FIG. 4 illustrates detailed RLC RESET procedure, considering UE as a sender and an UTRAN as a receiver. When a RESET condition occurs, the sender initiates a RESET procedure. Assuming that sender has UL HFN=x1 and DL HFN=y1 (401) and the receiver has UL HFN=x2 and DL HFN=y2 (402), then the sender sends RESET PDU to receiver with HFNI=x1 and, RSN=0 (403) and enters RESET_PENDING_STATE (404), in which the sender will stop sending and receiving data. Upon receiving this RESET PDU, receiver will send a RESET ACK PDU with RSN=0 and HFNI=y2 to sender via lower layers (405) and will RESET its state variables and update its UL HFN and DL HFN by making DL HFN=y2+1 and UL HFN=x1+1 (406).
Upon receiving the RESET ACK PDU sender will also reset its state variables, update DL HFN=y2+1 and UL HFN=x1+1 (407), and end the RESET_PENDING_STATE. Consequently, HFN of sender and receiver will be synchronized with UL HFN=x+1 and DL HFN=y+1.
As will be described below, the prior art suffers from the synchronization problem due to delayed arrival of RESET ACK PDU at sender, after single-sided re-establishment terminates.
FIG. 5 illustrates a scenario of a delayed RESET ACK PDU, after a single sided re-establishment. More specifically, FIG. 5 illustrates the scenario of delayed RESET ACK PDU, after single sided re-establishment, wherein the sender has initiated the RESET procedure and sends the RESET PDU with RSN=0 and HFNI=x1 (501). Further, receiver responds with RESET ACK PDU with RSN=0 and HFNI=y2 and submits it to lower layers (502). However, the RESET ACK PDU is delayed due to some reason, e.g., because of multiple HARQ retransmissions at MAC-hs (503). Single sided re-establishment (Transmission (TX) side of UE and Reception (RX) side of UTRAN) occurs and therefore, new values of the UL HFN are configured for TX side for UE and RX side for UTRAN (504).
As per 3GPP standards, if a single sided RLC re-establishment is initiated while an RLC RESET procedure is ongoing, then the RLC RESET procedure is aborted and the RLC RESET procedure is restarted after re-establishment terminates (505). In this new RLC RESET procedure, the RSN is set to zero. Wherein sender again sends RESET PDU with RSN=0 and HFNI=x3. Further, receiver responds with new RESET ACK PDU with RSN=0 and HFNI=y2+1 (506). Sender is in RESET_PENDING_STATE and when it receives the delayed RESET ACK PDU with RSN=0 and HFNI=y2, which was sent before re-establishment, it updates its HFN value accordingly and comes out of RESET_PENDING_STATE (507). Thereafter, when it receives the second RESET ACK PDU with RSN=0 and HFNI=y2+1, it discards it.
Consequently, DL HFN will be out of synchronization as at the sender side HFNs are with UL HFN=x3+1, DL HFN=y2+1 (507) and the receiver side with UL HFN=x3+1, DL HFN=y2+2 (508). Similarly, for the above-described scenario, DL HFN will be out of synchronization when single sided re-establishment (RX side of UE and TX side of UTRAN) happens (509).
As per the current standards, if double-sided RLC re-establishment is initiated while an RLC RESET procedure is ongoing, then the RLC RESET procedure is aborted and RLC RESET procedure is not restarted after re-establishment terminates. The sender entity after re-establishment, therefore, is not in RESET_PENDING_STATE and will discard this RESET ACK PDU. Consequently, the above-described HFN de-synchronization problem does not occur in the double-sided RLC re-establishment case.
Additionally, the prior art undergoes a synchronization problem due to delayed arrival of RESET PDU at receiver, after re-establishment terminates, in the double sided RLC re-establishment and in the single sided re-establishment.
FIG. 6 illustrates the scenario of delayed RESET PDU after double-sided re-establishment. In a double-sided RLC re-establishment (601), if the sender has already submitted RESET PDU to HARQ, before the RESET procedure is aborted, it is possible that this PDU is delivered to peer RLC entity, after the re-establishment has terminated. In such a scenario, the peer RLC entity will update its HFN according to received RESET PDU and respond with a RESET ACK PDU (602). However, the sender entity after re-establishment has not restarted the RESET procedure and therefore, is not in a RESET_PENDING_STATE and will merely discard this RESET ACK PDU (604). That is, the sender has already started transmitting and receiving data (603). Consequently, the HFNs of the sender and the receiver will not be in synchronization (605).
FIG. 7 illustrates a scenario of delayed RESET PDU after a single sided re-establishment. In case of single-sided re-establishment (701), because the sender has restarted the RESET procedure and transmitted a new RESET PDU (702), it would accept the RESET ACK PDU corresponding to delayed RESET PDU (703) and come out of RESET_PENDING_STATE (704). Hence, when the RESET ACK is received corresponding to new RESET PDU, it will be discarded (705). Hence, the sender and receiver will be out of synchronization (706).
U.S. Patent Application No. 2006/0281413, entitled “Establishing Radio Link Control in Wireless Communication Networks”, by Burbidge et al., describes a wireless communication terminal including a radio transceiver and a radio link control entity that re-initiates a radio link control RESET procedure when the transceiver receives a message containing an instruction to change an uplink or downlink protocol data unit size during an ongoing radio link control RESET procedure. For example, the radio access network may initiate an UL or DL PDU size change before terminal receives an acknowledgement that a previously initiated radio link control RESET procedure is completed. The radio link control entity restarts a radio link control RESET timer upon re-initiating the radio link control RESET procedure, after re-establishing radio link control.
Burbidge proposes a method and system for establishing and resetting radio link control in wireless communication networks. According to Burbidge, if a single-sided RLC establishment occurs during a pending RLC RESET procedure, the RLC RESET procedure is restarted. Further, the RSN sent in the RESET PDU, after the re-establishment is not initialized to zero, and instead, the RSN is set to a value of a last used RSN incremented by one. However, Burbidge does not explicitly mention toggling of the RSN value in the RESET PDU. Further, Burbidge does not disclose any other method for synchronizing HFN frames after re-establishment of an RLC entity. Therefore, a method is needed to addresses these issues and to avoid the de-synchronization of HFNs.